1. Field of the Invention
The present invention relates to a method of matching patterns and an apparatus therefore used in a checking system for checking the quality of offset sheet-fed printing or screen process printing performed on a molded product.
2. Description of the Related Art
A technique of checking the quality of a printed pattern or the like performed on a product by means of pattern matching is conventionally known, and this technique is performed as follows.
First, a product determined to be a good product by eye inspection or the like is prepared, and a to-be-checked object such as a printed pattern formed on the product is imaged by a television camera. An image signal from the camera is signal-processed, and outline data of the to-be-checked object is extracted. Enlarged outline data obtained by performing an enlargement process for the outline in the vertical and horizontal directions is stored as master pattern data in a memory. A slight error based on variations in to-be-checked objects or conveyance precision of products can be allowed by this enlargement process. Thereafter, a to-be-checked object of a to-be-checked product is imaged by the television camera. Outline data is extracted from an image signal corresponding to the to-be-checked object by an outline extraction circuit and compared with the master pattern data stored in the memory, thereby performing pattern matching. By such a conventional method of matching patterns, information for determining the quality of a to-be-checked object such as a printed pattern is obtained.
Defects encountered in a printed pattern are roughly classified into a short-defect and a splashing defect. In the short-defect, a portion of a printed pattern is omitted or becomes faint. In the splashing defect, splashed ink, dirt, or foreign matters are adhered on a portion other than a predetermined printed portion.
The above conventional technique is suitably used in detection of splashing defect information. Since, however, this conventional technique is based on the master pattern obtained by performing an enlargement process for the outline and an omitted portion of a printed pattern may be included in the master pattern, the technique is unsuitable to recognize short-defect information.